This disclosure relates generally to radiation apparatuses and methods, and in particular to multi level multileaf collimator (MLC) leaf tip effects and penumbra optimization using multi level MLCs.
Multileaf collimators are widely used in radiotherapy machines to support various treatments including intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). Conventional multileaf collimators include a single level of a plurality of beam blocking leaves arranged in two opposing banks or arrays. Each leaf in a bank is longitudinally movable relative to a leaf in the opposing bank. In operation, each of the individual leaves is positioned to block a portion of a radiation beam passing through the volume occupied by the leaf. The combined positioning of all leaves defines one or more apertures through which the unblocked radiation beam passes, and the aperture(s) define(s) the shape of the radiation beam directed to a treatment field in an isocenter plane.
It would be desirable to provide MLCs that can shape beams with high resolution so that the shaped beam conforms to a target volume as close as possible. Conventional single level MLCs have been designed to provide higher beam shaping resolution by making the beam blocking leaves thin. However, reducing the width of leaves to improve MLC resolution has limitations and imposes challenges to MLC construction and operation. For MLCs using screw leaf drive systems for example, long slender drive screws may be susceptible to column buckling in a way that scales dramatically worse with smaller screw diameters. Motors with a smaller diameter may also be required.
Conventional single level MLCs employs square leaf tips in a beam's eye view. Therefore, even with width resolution of 5-10 mm, conventional single level MLCs can only create a “stairstep” approximation of the desired treatment field.
Furthermore, conventional single level MLCs use single leaf tip design, which is generally optimized for penumbra performance for a small specific area in the treatment field but not for the other areas or large fields. Further, while single leaf tip design may provide optimal penumbra performance on a specific field contour, it may perform less desirably on various other contours due to the finite resolution of the leaf width and leaf tip geometry.